As a device for analyzing elements contained in a sample, an inductively coupled plasma mass spectrometer (ICP-MS) is known (e.g., Patent Literature 1). ICP-MS is advantageous in that a wide variety of elements from lithium to uranium (excluding some elements, such as a rare gas) can be analyzed at the ppt level (parts per trillion=1/1012), and has been used, for example, for quantifying various heavy metal elements contained in an environmental sample such as tap water, river water, or soil.
ICP-MS includes an ionization chamber equipped with an ICP ion source, which generates atomic ions from a sample (mainly a liquid sample) in an inductively coupled plasma, and a mass spectrometry unit that analyzes the generated ions. The ICP ion source includes a plasma torch having a sample tube through which a liquid sample nebulized by a nebulizer gas flows, a plasma gas tube surrounding the sample tube, a coolant gas tube further surrounding the plasma gas tube, and a high-frequency induction coil wound around a tip of the coolant gas tube. When a high-frequency current flows through the high-frequency induction coil of the plasma torch while a plasma gas (mainly argon gas) passes through the plasma gas tube, a plasma (plasma of as high as 6,000 to 10,000 K) is generated at the tip of the plasma torch. When a nebulized liquid sample is introduced into the sample gas tube in this state, compounds in the sample are atomized and ionized in the high-temperature plasma, resulting in the generation of atomic ions. The generated atomic ions are introduced into the mass spectrometry unit, separated according to the mass-to-charge ratio, and measured.
In many cases, in an ICP ion source, the sample is ionized with an argon gas plasma. Accordingly, in the ionization chamber, not only atomic ions but also polyatomic ions including argon added to atomic ions (argon adduct ions) are generated. When a mass-to-charge ratio of an argon adduct ions is close to the mass-to-charge ratio of the atomic ions to be analyzed, their mass peaks overlap on the mass spectrum. For example, a mass-to-charge ratio of Fe ions (atomic ions to be analyzed) is 55.934939, while a mass-to-charge ratio of ArO ions (argon adduct ions) is 55.957298, which are very close to each other, and thus their mass peaks overlap. The case of argon adduct ions has been described as an example here, but the same problem occurs also in other polyatomic ions besides the argon adduct ions. Hereinafter, atomic ions to be analyzed will be referred to as “analyte ions”, and ions having a mass peak overlapping a mass peak of the analyte ions are referred to as “interfering ions”.
Even in the case where the mass-to-charge ratio of analyte ions and that of interfering ions are close to each other as described above, both ions can be separated when the mass spectrometry unit has an enhanced mass resolution. As described above, the mass-to-charge ratio of Fe ions is 55.934939, and the mass-to-charge ratio of ArO is 55.957298. Accordingly, when a mass spectrometry unit having a mass resolution of two digits after the decimal point or higher is used, the mass peaks of these two kinds of ions can be separated. Examples of such mass spectrometry units include a time-of-flight mass spectrometry unit and a double-focusing mass spectrometry unit. These mass spectrometry units are expensive, and there also is a problem of increase in device size.
Accordingly, a method in which interfering ions are removed using a mass spectrometry unit equipped with a collision cell has been used. FIG. 1 shows an example of such a mass spectrometry unit. A mass spectrometry unit 130 includes, sequentially from a plasma torch 120 side, an interface unit 131, a converging lens 132, a collision cell 133, an energy barrier unit 134, a mass separation unit 135, and a detection unit 136.
Analyte ions and interfering ions generated in an ionization chamber both pass through the interface unit 131 and the converging lens 132 and are transported to the collision cell 133. An inert gas, such as helium gas, is introduced into the collision cell 133 from a gas supply source (not shown). The analyte ions and interfering ions that have entered the collision cell 133 both collide with inert gas molecules and lose their kinetic energy. The energy that the ions lose upon collision is represented by the following equation.
                    E        =                              E            ini                    ⁢                                                    m                1                2                            +                              m                2                2                                                                    (                                                      m                    1                                    +                                      m                    2                                                  )                            2                                                          [                  Equation          ⁢                                          ⁢          1                ]            
In the equation, E is an energy after a single collision, Eini is an initial energy (before the collision), m1 is the mass of an ion, and m2 is the mass of a collision gas molecule.
As is understood from the above equation, the amount of energy that an ion lose in a single collision varies depending on the mass of the ion. In other words, when an analyte ion and an interfering ion are similar in mass to each other, and also they have the same level of initial energy, the amount of energy that these ions lose in a single collision is almost the same. However, as compared to an analyte ion, which is a monatomic ion, a collision cross-section of an interfering ion, which is a polyatomic ion, is large. Accordingly, the average number of collisions with inert gas molecules during the passage through the collision cell 133 is large in interfering ions, and kinetic energy of interfering ions after passing through the collision cell 133 is smaller than kinetic energy of analyte ions. Therefore, by properly adjusting the height of the energy barrier set downstream from the collision cell 133, it is possible to pass analyte ions with large kinetic energy, while blocking interfering ions with small kinetic energy. The technique of separating analyte ions and interfering ions utilizing the difference in energy loss of ions upon gas collision in this manner is referred to as a kinetic energy discrimination (KED) method (e.g., Non Patent Literature 1).
The KED method described above is effective in blocking interfering ions; however, some of analyte ions are also lost. In particular, atomic ions having a small mass lose a large amount of kinetic energy in a single collision, and also the ion flight direction is more likely to change upon collision. As a result, the amount of ions exiting from the collision cell and passing through the energy barrier decreases, resulting in a decrease in measurement sensitivity. Then, conventionally, when measuring middle- to large-mass atomic ions, or when atomic ions to be analyzed are generated in a large amount, an analysis in which a gas is introduced into the collision cell to preferentially block interfering ions (with-gas analysis) is performed. Meanwhile, when measuring low-mass atomic ions, or when focus is placed on the measurement sensitivity, an analysis in which no gas is introduced (without-gas analysis) is performed. Particularly in the case where the analyte ions are atomic ions whose mass-to-charge ratio is equal to or lower than that of argon, argon adduct ions do not act as interfering ions to such analyte ions, and thus the high-sensitivity, without-gas analysis is suitable.